1. Field of the Invention
The present invention relates to an apparatus which homogenizes the energy distribution of a laser beam in a certain specified area. It relates also to a method for the homogenization. In addition, the present invention relates to a semiconductor device which has a circuit including a thin film transistor fabricated by the use of means for the homogenization. By way of example, it relates to the constructions of an electro-optical device which is typified by a liquid crystal display device, and an electric equipment in which such an electro-optical device is installed as a component. Incidentally, here in this specification, the expression “semiconductor device” shall signify general devices which can function by utilizing semiconductor properties, and it shall cover the electro-optical device and electric equipment mentioned above.
2. Description of the Related Art
In recent years, researches have been extensively made on techniques wherein an amorphous semiconductor film or a crystalline semiconductor film (namely, a semiconductor film having the crystallinity of a poly-crystalline, a micro-crystalline or the like different from a single crystal), that is, a non-single crystalline semiconductor film which is formed on an insulating substrate of glass or the like is subjected to laser annealing, thereby to be crystallized or to enhance the crystallinity. A silicon film is often employed as the semiconductor film.
As compared with a quartz substrate having heretofore been often used, the glass substrate is inexpensive and is rich in machinability, and hence, it has the merit of being capable of the easy manufacture of a large area substrate. Therefore, the researches are vigorously made. The reason why a laser is used for the crystallization by preference, is that the melting point of the glass substrate is low. The laser can give high energy to the non-single crystalline semiconductor film only without considerably changing the temperature of the substrate.
A crystalline silicon film formed by performing the laser annealing has a high mobility. Therefore, thin film transistors (TFTs) are formed by employing the crystalline silicon film, and they are actively utilized for, for example, a liquid-crystal electro-optical device of monolithic type in which the TFTs for pixel drive and for driver circuits are fabricated on one glass substrate. Since the crystalline silicon film is made up of a large number of crystal grains, it is called a “poly-crystalline silicon film” or “poly-crystalline semiconductor film”.
Besides, a method wherein a beam from a pulsed lasing type laser of high power, such as excimer laser, is worked by an optical system so as to define a tetragonal spot having a size of several (cm square) or a line having a length of at least 10 (cm) on a surface to-be-irradiated, and the surface to-be-irradiated is scanned by the worked laser beam (in other words, the projected position of the laser beam is moved relatively to the surface to-be-irradiated), thereby to carry out the laser annealing, is preferably used because it is excellent in industry owing to a good mass-productivity.
Especially with the rectilinear laser beam, the whole surface to-be-irradiated can be irradiated with the laser beam by the scanning in only a direction orthogonal to the lengthwise direction of this rectilinear laser beam, unlike in the case of the spot-like laser beam with which the scanning needs to be done in the lengthwise and widthwise directions of the surface to-be-irradiated, so that the high mass-productivity is attained by the annealing with the rectilinear laser beam. The scanning of the surface to-be-irradiated in the direction orthogonal to the lengthwise direction of the rectilinear laser beam is done for the reason that the scanning direction is the most efficient. Owing to the high mass-productivity, it is currently becoming the mainstream to use for the laser annealing the rectilinear laser beam which is obtained in such a way that the laser beam of the pulsed lasing type excimer laser is worked by the appropriate optical system.
FIGS. 2A and 2B show an example of an optical system for working the cross section of a laser beam into the shape of a line on a surface to-be-irradiated. The optical system shown in FIGS. 2A and 2B is very common. The optical system functions, not only to change the cross-sectional shape of the laser beam into the rectilinear shape, but also to homogenize the energy of the laser beam on the surface to-be-irradiated. In general, an optical system which homogenizes the energy of a beam is called a “beam homogenizer”. The optical system shown in FIGS. 2A and 2B is also the beam homogenizer.
When an excimer laser emitting ultraviolet radiation is used as a light source, the constituent material of the optical system may be, for example, quartz entirely. This is because a high transmission factor is attained. Besides, coatings should favorably be ones capable of attaining transmission factors of at least 99 (%) for the wavelengths of the excimer laser employed.
First, a side view in FIG. 2A will be referred to. A laser beam emergent from a laser oscillator 1201 is split in a direction orthogonal to the traveling direction of the laser beam by cylindrical lens arrays 1202a and 1202b. Here in this specification, the orthogonal direction shall be termed the “vertical direction”. When a mirror is incorporated midway of the optical system, the vertical direction curves into the direction of light bent by the mirror. In the illustrated construction, the emergent laser beam is split into four. The split laser beams are once brought together into one laser beam by a cylindrical lens 1204. The laser beam is split again, and the resulting laser beams are reflected by a mirror 1207. Thereafter, the reflected laser beams are condensed into one laser beam again on a surface to-be-irradiated 1209 by a doublet cylindrical lens 1208. The expression “doublet cylindrical lens” signifies a lens which is made up of two cylindrical lenses. Thus, the energy of the rectilinear laser beam in the widthwise direction thereof is homogenized, and the width of the rectilinear laser beam is determined.
Next, a top view in FIG. 2B will be referred to. A laser beam emergent from a laser oscillator 1201 is split in a direction which is orthogonal to the traveling direction of the laser beam and which is also orthogonal to the vertical direction explained above, by a cylindrical lens array 1203. Here in this specification, the orthogonal direction shall be termed the “lateral direction”. When a mirror is incorporated midway of the optical system, the lateral direction curves into the direction of light bent by the mirror. In the illustrated construction, the emergent laser beam is split into seven. Thereafter, the split laser beams are combined into one laser beam on a surface to-be-irradiated 1209 by a cylindrical lens 1205. A mirror 1207, et seq. are depicted by broken lines, which indicate exact optical paths and the exact positions of a lens 1208 and the surface to-be-irradiated 1209 in the case where the mirror 1207 is not arranged. Thus, the energy of the rectilinear laser beam in the lengthwise direction thereof is homogenized, and the length of the rectilinear laser beam in the lengthwise direction is determined.
As described above, the cylindrical lens arrays 1202a and 1202b or the cylindrical lens array 1203 serve(s) as lens means for splitting the laser beam. The homogeneity of the laser beam to be attained is determined by the number of the splitting (split laser beams).
The lenses mentioned above are made of synthetic quartz in order to conform to an excimer laser. Besides, the surfaces of the lenses are formed with coatings in order to efficiently transmit the beams of the excimer laser. Thus, a transmission factor for the excimer laser beam(s) per lens has reached at least 99 (%).
The rectilinear laser beam worked by the above construction is projected overlappingly while being gradually shifted in the widthwise direction of the laser beam, whereby the whole surface of a non-single crystalline silicon film, for example, can be subjected to laser annealing so as to be crystallized or to enhance the crystallinity thereof.
Now, there will be described a typical example of a method of fabricating a semiconductor film to-be-irradiated.
First, a “Corning 1737 Glass” substrate (manufactured by Corning Incorporated) being of 0.7 (mm) in thickness and 5 (inches square) was prepared. Using a plasma CVD equipment, an SiO2 film (silicon oxide film) having a thickness of 200 (nm) was formed on the substrate, and an amorphous silicon film (hereinbelow, expressed as “a-Si film”) having a thickness of 50 (nm) was formed on the surface of the SiO2 film. The resulting substrate was heated at a temperature of 500 (° C.) in a nitrogen atmosphere for one (hour), thereby to lower the hydrogen content of the a-Si film. Thus, the tolerance of the a-Si film to a laser was remarkably enhanced.
An XeCl excimer laser (having a lasing wavelength of 308 (nm) and a pulse width of 30 (ns)) “L3308” manufactured by Lambda Physik, Inc. was used as a laser oscillator. The laser oscillator emits a pulsed laser beam, and is capable of producing an energy level of 500 (mJ) per pulse. The size of the laser beam is 10 (mm)×30 (mm) (both being in terms of half-value widths) at the exit of this laser beam. Here in this specification, the exit of the laser beam shall be defined on a plane perpendicular to the traveling direction of this laser beam as viewed immediately after the emergence thereof from the laser oscillator.
In general, the shape of a laser beam emitted from an excimer laser is a rectangle, and it falls within a range of about 1-5 in terms of an aspect ratio. The intensity of the laser beam demonstrates a Gaussian distribution where it is higher as the center of the laser beam comes nearer. The laser beam in the example was changed into a rectilinear laser beam having a uniform energy distribution and a size of 125 (mm)×0.4 (mm), by the optical system shown in FIGS. 2A and 2B.
According to the inventor's experiment, in the case of irradiating the above semiconductor film with the laser beam, about {fraction (1/10)} of the width (half-value width) of the rectilinear laser beam was the most suitable as the pitch of the overlap. Thus, the homogeneity of the crystalline semiconductor film was enhanced. Since the half-value width was 0.4 (mm) in the example, the laser beam was projected by setting the pulse frequency of the excimer laser at 30 [Hz] and the scanning speed thereof at 1.0 [mm/s]. On this occasion, an energy density on the surface to be irradiated with the laser beam was set at 400 (mJ/cm2). The method thus far explained is a very common one which is employed for crystallizing the semiconductor film by using the rectilinear laser beam.
Regarding the optical system shown in FIGS. 2A and 2B, high machining precisions are required of the cylindrical lens arrays, the cylindrical lenses and the doublet cylindrical lens. Besides, since the large number of lenses are employed, the positional adjustments among them are difficult. Therefore, a desired beam can be obtained for the first time when a considerably skilled operator makes the adjustments. In addition, since the optical system is mainly constructed of optical lenses, the deteriorations of the optical lenses attributed to the laser beams are inevitable.
By way of example, in case of employing a KrF excimer laser (at a lasing wavelength of 248 (nm)) as a light source, even when lenses made of quartz of excimer grade are adopted as the optical lenses, they have lifetimes of, at most, several years, and are very costly in consideration of the price of the optical system.
When the optical system has deteriorated, the overall transmission factor thereof lowers chiefly. This is a serious problem especially in a process for crystallizing a semiconductor film as requires high power.
Moreover, in recent years, the areas of substrates have been remarkably enlarged in order to enhance productivities. The sizes of substrates which are being developed anew and which are to be dealt with in a mass-producing plant, are being standardized as at least 600 (mm)×720 (mm). Consequently, the length of a rectilinear laser beam needs to be, at least, equal to that of the shorter latus of each of the substrates. Since the rectilinear laser beam of such a length can laser-anneal the whole surface of one large-area substrate by one time of scanning, it affords an excellent productivity and is very useful.
In contrast, in a case where the whole surface of one large-area substrate is to be laser-annealed using a rectilinear laser beam the length of which is less than that of the shorter latus of the large-area substrate, the rectilinear laser beam must be scanned a plurality of number of times. With such laser annealing, a semiconductor device cannot be fabricated at the boundary between, for example, a substrate part crystallized by the first laser-annealing scanning and a substrate part crystallized by the second laser-annealing scanning. Even if the semiconductor device is fabricated at the boundary, no satisfactory characteristics may possibly be attained.
However, when the rectilinear laser beam whose length is at least 600 (mm) is to be defined using the optical system of the prior-art example as it is, the size of the doublet cylindrical lens 1208 approximates to 600 (mm). A lens having such a size is enormously expensive, (nearly hundred million yen) and also the deterioration of the lens takes place, so that the lens is difficult of incarnation in practical use.